Skid for transporting a nuclear fuel transportation cask

ABSTRACT

A skid for transporting a nuclear fuel transportation cask. The skid comprises a supporting member having parallel spaced-apart plates. The plates are aligned perpendicular to a longitudinal axis of the cask, include a semi-circular trough for mating with the cask, and are connected by longitudinal fins parallel to the longitudinal axis of the cask. The skid also comprises a retaining member including parallel spaced-apart plates. The plates of the retaining member are aligned perpendicular to a longitudinal axis of the cask, include a trough for mating with the cask, and are connected by longitudinal fins parallel to the longitudinal axis of the cask. The fins of the supporting member are spaced apart such that when the cask rests in the trough of the supporting member, the fins of the supporting member are aligned with elongate members of the cask neutron radiation shielding material to transfer the load between the cask and the skid.

This is a divisional of the prior application Ser. No. 08/131,973, filedon Oct. 8, 1993, of Kyle B. Jones, Robert E. Lehnert, Ian D. McInnes,Robert D. Quinn, Steven E. Sisley, and Charles J. Temus forTRANSPORTATION AND STORAGE CASK FOR SPENT NUCLEAR FUEL, now U.S. Pat.No. 5,406,600, the benefit of the filing date of which are herebyclaimed under 35 U.S.C. §120.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to containers used for transportation andshort term storage of spent nuclear fuel.

BACKGROUND OF THE INVENTION

As the nuclear utility industry matures, there is an ever-increasingneed for additional storage space to safely contain spent nuclear fuel.One method that has been developed in recent years for storage of spentnuclear fuels is dry storage in horizontal storage modules, which areshielded bunkers in which containerized spent fuel is stored andmonitored for definite periods of time. One conventional technique forhorizontal modular dry storage of spent nuclear fuel rods is disclosedin U.S. Pat. No. 4,780,269 to Fischer et al.

A basic procedure for dry storage of spent nuclear fuel is to position adry shielded canister into a shielded transfer cask. The canister andcask are filled with deionized water, which is then lowered into a poolcontaining the spent nuclear fuel. Spent fuel assemblies are then placedinto the canister, and a shielded end plug is positioned to close thecanister. The canister and cask are then removed from the pool, and thecask and canister are drained and dried. The exterior of the cask isdecontaminated, followed by closure of the cask with a closure plate.The closed transportation cask is then lowered onto a transport trailerand secured by tie-downs.

The transport trailer carries the cask to the sight of the horizontaldry storage modules. The cask is opened and docked with an entry port ofa dry storage module. The canister is then transferred from the caskinto the module, such as by passing a ram through the dry storage modulefrom an end opposite the entry port, through the entry port and into theopened cask. The canister can then be grasped and pulled into the drystorage module, after which both the entry port and access port aresealed.

A critical aspect of this process is the safe containment and transferof the spent nuclear fuel within the canister from the original poolstorage to the final dry horizontal storage site. The transport caskmust be constructed with adequate structural strength and shielding toboth physically protect the dry shielded canister within, and to providebiological shielding to minimize personnel radiation dosages duringcanister transfer and transport operations.

During the canister transfer and transport process, the cask must beable to withstand any foreseeable impact, such as could occur byaccidental dropping of the cask from the transport trailer or exposureto tornadoes or other natural disasters. In the United States, federalregulations setting forth requirements that transport casks must meetare found in 10 C.F.R. 72, including subpart G, as well as 10 C.F.R. 71and 10 C.F.R. 50. In particular, the cask must be able to withstandimpacts due to a drop of 30' onto an essentially, unyielding flathorizontal surface, without structural failure. Even if structurallydamaged, no leakage of the contents from the cask is permitted.

It is thus important to design casks with high structural integrity. Atthe same time, it is desirable to maximize the quantity of spent fuelthat can be transported within the cask at any given time, and tominimize the cost of constructing the cask. While strengthconsiderations typically warrant constructing the cask from thickersections of metal and other materials, this requirement may reduce thequantity of spent fuel that can be transported within the cask. Externaldimensions of the cask are limited by constraints such as the totalweight of the loaded cask, and clearances required to transport thecasks through tunnels, under bridges and overpasses, and the like.

Currently, conventional casks are often constructed from a polishedaustentitic stainless steel, such as 304 stainless steel, for corrosionprevention. However, such stainless steel is limited in strength and mayfail under high stresses. To combat this potential, conventional casksare constructed from thick metal sections, and must be reinforced withgusset plates and other reinforcing members. Additionally, locations onthe casks that are subjected to force during transport must bereinforced with additional metal plates welded to the cask structure.

For example, conventional casks are outfitted with cylindrical trunnionswelded or bolted directly to a cylindrical structural shell of the caskat diametrically opposed locations. These trunnions are grasped byhooks, and serve as pivot points while lieting the cask during thetransportation process. Because of the stresses transferred to the caskstructure from the trunnions during use, the shell is typicallyreinforced in the area surrounding the trunnions by welding additionalplates of metal.

The trunnions themselves are conventionally permanently secured to thestructural shell of casks by welding or bolting directly to the shell.In the case of welding, the welded joint is subjected to substantialstress during hoisting of the cask. In the case of bolting the trunnionsin place, the bolts are subjected to extreme shear and tensile loadsduring hoisting of the cask. Again, the trunnions must be heavilyreinforced to withstand such loads, increasing the weight and overalldimensions of the cask, and thus decreasing the spent fuel containmentcapacity and increasing the cost of manufacture.

When sealed joints, such as elastomeric (e.g., O-ring) seals or metalseals are utilized, the base metal used to form the structural shell isconventionally machined to form the sealing surfaces. Thus, for example,when 304 stainless steel is used to construct the shell, annularsurfaces on the shell are machined and polished to form sealingsurfaces. While functioning adequately in most situations, extremeimpact to the seal area, such as by accidental dropping of the cask atan oblique angle whereby force is concentrated on the seal area, mayresult in permanent deformation of the metal seal surface, andsubsequent leakage potential.

SUMMARY OF THE INVENTION

The present invention provides a container designed for use as a caskfor short-term containment and transporting of spent nuclear fuel. Inthe first aspect of the present invention, the container is formed froma structural shell defining a cavity for receiving spent nuclear fuel,and first and second end apertures opening into the cavity. The shellhas a first end portion formed of a first material and a second endportion formed of a second material. The first end portion is joined tothe second end portion to form the structural shell. A bearing surfaceis defined on the first end portion of the shell and is engageable toenable hoisting of the container. The first end portion of the shell isconstructed from a first material that has a higher load bearingstrength than the second material, to handle the hoisting stress. Thecontainer also includes a first closure securable to the first endportion of the shell to seal the first end aperture, and a secondclosure securable to the second end portion of the shell to seal thesecond end aperture. The container further includes a radiationabsorbing shield layer, which may include both gamma radiation and aneutron radiation absorbing materials.

The container is thus constructed so that those areas of the containerthat are subjected to the greatest stress, e.g. the first end portion,is constructed from the strongest material, such as a high-strengthmetal alloy. However, those portions of the cask that are not exposed toas high a stress are produced from lower cost materials having astrength that is adequate for the lower loads to be imposed on thoseportions.

In a further aspect of the present invention, a cask is provided thatincludes a tubular inner shell defining a cavity for receiving spentnuclear fuel, and first and second ends. A tubular outer shell havingfirst and second ends is assembled coaxially over the inner shell todefine an annular space therebetween. A radiation absorbing materialfills the annular space. An annular member defining a central apertureand a first annular sealing surface is secured about its perimeter tothe first ends of the inner shell and the outer shell to create airtightjoints with both the inner shell and the outer shell. A first closureplate is releasably securable to the annular member and defines a secondannular sealing surface corresponding to the first annular sealingsurface defined by the annular member. A seal is positioned between thesecond annular sealing surface of the first closure plate and the firstannular sealing surface of the annular member to create an airtight sealbetween the first closure plate and the annular member. The cask alsoincludes a second closure plate secured proximate its perimeter to thesecond ends of the inner shell to create airtight joints with the innershell.

In a further aspect of the present invention, a cask is provided thatincludes a structural shell defining a cavity for receiving spentnuclear fuel and first and second end apertures. A first closure issecurable to the shell to seal the first end aperture. A second closureis securable to the shell to seal the second end aperture. A radiationabsorbing shield layer is affixed to the shell. First and second pairsof trunnion mounting structures, preferably configured as tubularsleeves are secured in opposing disposition within apertures formed inthe structural shell. First and second trunnions, each defining a baseand a beating surface, are included. The base of each trunnion isreleasably securable to a corresponding one of the trunnion mountingstructures, whereby the bearing surfaces of the first trunnions can begrasped to hoist the container. The second trunnions are used to providea point of support and rotation for loading and unloading the cask fromits conveyance.

In a preferred embodiment, the trunnion mounting structures areconfigured as annular sleeves that are welded to the structural shell ofthe cask, within which sleeves the base of the trunnions are received.Because of this construction, fasteners such as bolts used to secure thetrunnions to the mounting structures are substantially isolated fromtensile and shear loads.

In a further aspect of the present invention, the trunnion mountingstructures are preferably formed from a high-strength material such asis used to form the portion of the outer shell to which the firsttrunnions are mounted, thereby providing a strong trunnion mountingwithout requiting additional plate reinforcement.

In a still further aspect of the present invention, improved seal jointsare included in the cask. Sealing surfaces of the cask are formedutilizing hardened metal weld overlays, thereby providing sealingsurfaces that are not readily subject to permanent deformation uponimpact of the cask. In the preferred embodiment, sealing surfaces ofclosure plates on the cask include grooves formed to define ahalf-dovetailed cross section for receiving seals. This enables use ofeither metal or elastomeric seals in the joints, and enables assembly ofthe joints while the cask is in either the horizontal or verticaldisposition.

In a still further aspect of the present invention, a cask is disclosedthat includes a tubular structural shell defining a cavity for receivingspent nuclear fuel and first and second opened ends. The first closureplate is releasably securable to the first opened end of the shell,whereby when secured to the shell, the first opened end of the shell issealed, and when released from the shell, loading and unloading of spentnuclear fuel through the first open end into the cavity is permitted.The second closure plate is secured to and seals the second open end ofthe shell. The second closure plate defines a central access aperture.An access cover plate is releasably securable to the second closureplate to seal the central access aperture. When released from the secondclosure plate, entry of a ram through the access aperture into thecavity of the shell to facilitate unloading of spent nuclear fuelthrough the first open end of the shell is permitted.

Shield plugs filled with a radiation-absorbing material are provided tocover the trunnion mounting structures and central access apertureformed in the cask during short-term storage and transportation.

In another aspect, the present invention relates to a skid fortransporting a nuclear fuel transportation cask and containment vessel.The skid supports the cask around the neutron radiation shieldingmaterial. The skid includes a supporting member and a retaining memberthat each include a plurality of parallel spaced-apart plates lying inplanes perpendicular to a longitudinal axis of the cask which areconnected by a plurality of longitudinal fins parallel to thelongitudinal axis of the cask. The longitudinal fins are positioned tomate with structural elements associated with the neutron radiationshielding material to transfer loads from the cask to the skid.

The present invention thus provides a cask that is less costly toconstruct, yet that provides improved safety under impact conditions.Exposure of workers to radiation during transport procedures is alsoreduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention and its attendant advantages will be more readilyunderstood in view of the following description, when read inconjunction with the accompanying drawings, in which:

FIG. 1 provides an isometric view of a cask constructed in accordancewith the present invention, with a portion of the cask cut away to showa dry shielded canister for containment of spent nuclear fuel installedtherewithin, and with a shield plug for a tie-down key way shownexploded from the cask;

FIG. 2 provides a cross-sectional view of the cask of FIG. 1, excludingthe internal dry shielded canister, taken along a plane intersecting thelongitudinal axis of the cask as indicated by line 2-2 in FIG. 1, withan access aperture closure plate and top closure plate shown explodedfrom the cask;

FIG. 3A provides a detailed cross-sectional view of the mated topclosure plate and top end of the cask body of FIG. 2, taken along thesame plane as in FIG. 2;

FIG. 3B provides a still further detailed cross-sectional view of thetop closure plate sealing surfaces shown in FIG. 3A;

FIG. 4 provides a detailed cross-sectional view of the welded bottomplate closure and assembled access aperture closure plate, taken alongthe same plane as in FIG. 2;

FIG. 5A provides a detailed cross-sectional view of the ram closureplate exploded from mating surfaces of the bottom closure plate, takenalong the same plane as in FIG. 2;

FIG. 5B provides a still further detailed cross-sectional view of theram closure plate sealing area, as shown in FIG. 5A;

FIG. 6 provides an end view of the cask of FIG. 1 to illustrate the"top" end of the cask, with a partial cross section taken along a planeoriented orthogonally to the longitudinal axis of the cask to show theinternal construction of the neutron shield jacket and cask body;

FIG. 7 provides a detailed cross-sectional view of the shear key waystructure of the cask of FIG. 1, taken along the same plane as in FIG.2;

FIG. 8 shows a detailed cross-sectional view of the shear key waystructure of the cask of FIG. 1, taken along a plane orientedorthogonally to the longitudinal axis of the cask;

FIG. 9 provides a detailed exploded view of the upper trunnion andtrunnion mounting sleeve of the cask of FIG. 1, taken along a planeoriented orthogonally to the longitudinal axis of the cask and alignedwith the central axis of the upper trunnion;

FIG. 10 provides a detailed exploded view of the lower trunnion andtrunnion mounting sleeve of the cask of FIG. 1, taken along a planeoriented orthogonally to the longitudinal axis of the cask and alignedwith the central axis of the bottom trunnion;

FIG. 11A provides an elevation view of the cask of FIG. 1, to illustratethe long side of the cask, with shield plugs installed to cover thetrunnion mounting sleeves and the access aperture port, and with theshear key way structure being shown in cross section to illustrateplacement of the key way shield plug;

FIG. 11B provides a detailed side elevation view of the access apertureshield plug assembly, shown in partial cross section taken along a planealigned with the longitudinal axis of the cask;

FIG. 12 is an environmental view of a transportation cask protected byimpact limiters and carded by a skid formed in accordance with thepresent invention resting on a conventional trailer;

FIG. 13 is a perspective view of the skid in FIG. 12; and

FIG. 14 is a front view of the skid in FIG. 12.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of a cask 10 for transportation and short-termstorage of spent nuclear fuel is shown in FIG. 1. The cask 10 includes abody 12 constructed from a tubular structural shell 14 having an uppershell portion 16 and a lower shell portion 18. The lower shell portion18 is sealed by a bottom closure plate 20 that has a central accessaperture 22 that is sealed with an access cover plate 24. The uppershell portion 16 is sealed with a top closure plate 26. The exterior ofthe structural shell 14 is shielded with a neutron absorbing shieldjacket 28. Two diametrically opposed upper trunnions 30 (only one shown)are secured within upper trunnion mounting sleeves 32 to the exterior ofthe upper shell portion 16. Two lower trunnions 34 are secured indiametric opposition to the lower shell portion 18 within lower trunnionmounting sleeves 36.

As used herein throughout, "bottom" and "lower" refer to the end of thecask 10 and its components closest in proximity to the bottom closureplate 20. while the words "top" and "upper" refer to the opposite endproximate the top closure plate 26. A dry storage canister 38 for spentnuclear fuel is shown installed within the interior cavity 40 of thecask 10. The construction of the dry storage canister 38 is fullydescribed in a U.S. Pat. No. 5,438,597, the disclosure of which ishereby expressly incorporated by reference.

A plurality of lugs 42 are secured to the structural shell 14 and to theannular ends of the shield jacket 28 about the circumference of the cask10, on both the lower and upper (not shown) ends of the shield jacket28. The purpose of the lugs 42 is to enable mating of the cask 10 withimpact limiters during transport. Impact limiters and transportationskids suitable for use in transporting the cask 10 are fully disclosedin a U.S. Pat. No. 5,394,449, the disclosure of which is herebyexpressly incorporated by reference.

Referring now to FIG. 2, the construction of the body 12 shall bedescribed. The body 12 has an overall cylindrical configuration andincludes the structural shell 14. The structural shell 14 has a tubularconfiguration and defines a central longitudinal axis 44 that is alignedwith the central longitudinal axes of the other annular components ofthe body 12, as shall be described. The lower shell portion 18 of thestructural shell 14 also has a tubular configuration, defining acircumferential bottom edge 46 and a circumferential top edge 48. Thelength of the lower shell portion 18 is approximately two-thirds thelength of the structural shell 14. The upper shell portion 16 extendsthe remaining one-third of the length, and defines a circumferentiallower edge 50 and a circumferential top edge 52. The upper edge 48 ofthe lower shell portion 18 abuts and is welded to the lower edge 50 ofthe upper shell portion 16, using a full penetration weld around theentire circumference of the structural shell 14. The upper shell portion16 and lower shell portion 18 each have a central axis that is alignedwith the longitudinal axis 44, and cooperatively define a rightcylinder.

The lower shell portion 18 is formed from a rigid material, preferably acorrosion resistant metal, and most preferably a stainless steel, suchas ASME SA-240 type 304 austentitic stainless steel. However, the uppershell portion 16 is preferably formed from a material having a higherload beating strength, also preferably a stainless steel, such as ASMESA-240 type XM-19 high alloy stainless steel. Type XM 19 stainless steelis also austentitic, but has approximately twice the load bearingstrength of type 304.

As shown in FIG. 1, the upper trunnions 30 are secured to the uppershell portion 16. The upper trunnions 30 are intended to be used forhoisting and lifting the cask 10, both when empty and when loaded with afull canister 38. Thus, the upper trunnions 30 in use transmitsignificant shear and tensile loads to the upper shell portion 16. Thelower shell portion 18 carries the lower trunnions 34, which are used toupright and stabilize the cask 10 during transport, as shall bedescribed subsequently, and thus are subjected to lower loading. Becausetype XM-19 stainless steel is more costly than type 304 stainless steel,the cost of manufacture is reduced by utilizing the XM-19 for the loadbeating portions of the cask 10. Both portions of the structural shell14 can be formed and welded from rolled plate.

Referring to FIG. 2, coaxially installed within the structural shell 14is an inner shell 54, which also may be formed film type 304 stainlesssteel or other suitable corrosion resistant structural materials. Theinner shell 54 is slightly smaller in external diameter than theinterior of the structural shell 14, and thus defines an annular spacetherebetween. This annular space is filled with a gamma radiationabsorbing material 56, such as ASTM B-29 chemical lead. The steelcontained in the structural shell 14 and the inner shell 54, as well asthe bottom closure plate 20 and top closure plate 26, also serve toabsorb gamma radiation.

The shield jacket 28 has a tubular configuration and is installed overand surrounds the majority of the length of the structural shell 14. Theshield jacket 28 is formed from a tubular outer skin 58. The internaldiameter of the outer skin 58 is greater than the external diameter ofthe shell 14, thus defining an annular space that is filled with aneutron radiation absorbing shield material 60. One suitable neutronradiation absorbing shield material 60 is a hydrogenous solid neutronabsorbing material, such as a cementious castable neutron absorbingmaterial. The upper and lower ends of the shield jacket 28 are closed byupper and lower annular support rings 62 and 63, respectively, welded tothe exterior of the structural shell 14 and the edges of the outer skin58. The lower annular support right 63 includes stainless steel rupturediscs which prevent over pressurization of the shield jacket 28.

A pair of elongate rails 64 are secured by welding or other means to theinterior of the inner shell 54 of the body 12. The rails 64 are orientedparallel to the central longitudinal axis 44 of the cask 10, and extendthe length of the inner shell 54. Each rail 64, also shown in FIG. 6, isformed from a strip of flat sheet. The rails 64 are spaced radiallyapart from each other within the same radial quadrant of the inner shell54. The rails 64 are positioned on the side of the cask body 12 thatrests on the trailer or other support surface when the cask 10 is laiddown horizontally

Each rail 64 is preferably formed from a material that is harder thanthe material used to construct the inner shell 54, such as a hardenedstainless steel, which provides a non-gouging, low-friction surface forthe canister 38 to slide on during installation or removal of thecanister 38 from the cask 10. One suitable material is nitronic 60, coldreduced sheet, ASTM A-240, grade UNS, 521800, RC29.35 stainless steel.

The bottom edge 46 of the lower shell portion 18 and the bottom edge ofthe inner shell 54 are each welded to the bottom closure plate 20,thereby sealing the bottom end of the body 12, as shall be described inmore detail subsequently. The top edge 52 of the upper shell portion 16and the top edge of the inner shell 54 are each welded to an annularsealing ring 66. The top closure plate 26 can be secured to the annularsealing ring 66 to selectively close the top end of the body 12.

Reference will now be had to FIGS. 2 and 3A to describe theconfiguration of the annular sealing ring 66. The sealing ring 66 has amain body portion having an essentially rectangular cross section. Anannular lower flange 70 extends downwardly from the lower surface of thebody portion 68 adjacent the inner edge of the ring 66. The lower flange70 has an internal diameter substantially equal to the internal diameterof the internal shell 54.

The top edge 52 of the upper shell portion 16 is welded to the main bodyportion 68 of the annular sealing ring 66, while the lower edge of thelower flange 70 is welded to the top edge of the inner shell 54. Bothwelds are full penetration welds extending around the full circumferenceof the annular sealing ring 66. The top surface of the body portion 68defines an annular abutment surface 74. An annular upper flange 76projects upwardly from the abutment surface 74 along the outer perimeterof the annular sealing ring 66.

A hardened sealing surface is formed on the abutment surface 74 by anannular hardened metal inlay 78. The inlay 78 is preferably formed byweld overlay of a hard metal onto the base metal of the annular sealingring 66. The annular sealing ring 66 is preferably formed from amachined ring forging of type 304 stainless steel. The inlay 78 ispreferably formed of inconel alloy. The inlay 78 wraps the inner uppercorner of the body portion 68 of the annular sealing ring 66, so that itprovides a hard polished surface on both the inner portion of theabutment surface 74 and the upper portion of the internal diameter ofthe body portion 68. The hard surface provided by the inlay 78 is highlyresistant to permanent deformation upon impact of the joint area of thecask 10.

Referring still to FIGS. 2 and 3A, the top closure plate 26 isconfigured as a solid disk. The top plate 26 has an annular recessformed about its perimeter in its bottom side that defines an annularsealing surface 80. The annular sealing surface 80 corresponds indimension substantially to the abutment surface 74 of the annularsealing ting 66. As shown in FIG. 3 A, the top closure plate 26 isinstalled on the body 12 by sliding the top closure plate 26 within theannular upper flange 76 of the annular sealing ting 66. When soinstalled, the sealing surface 80 of the top closure plate 26 abuts theabutment surface 74 of the annular sealing ting 76. A non-recessedcenter portion 82 of the bottom side of the top closure plate 26 isreceived within the inside diameter of the body portion 68 of theannular sealing ring 66.

The inlay 78 provides the sealing surface for the annular sealing ring66. Two annular grooves 84 are formed in the portion of the sealingsurface 80 of the top closure plate 26 that overlies the inlay 78. Asshown in FIG. 3A, a seal 86 is received within each of the grooves 84.The seals may be either deformable metal seals, or elastomefic seals,e.g., O-rings, or alternately configured elastomeric seals. The seals 86are deformed between the top closure plate 26 and the annular sealingring 66, and retained within the grooves 84.

Referring to FIG. 3B, each of the grooves 84 defines a half-dovetailcross-section, having a bottom surface 88, a first orthogonal sidesurface 90, and a second, inwardly angled side surface 92. Thehalf-dovetail configuration of the grooves 84 ensures that the seals 86are retained within the grooves 84 when the body 12 is positioned eitherhorizontally or vertically and the top closure plate 26 is removed.

The weld between the annular sealing ring 68 and the inner shell 54 isairtight. The weld between the annual sealing ring 68 and the uppershell portion 16 is also believed to be airtight, but is not tested forthat characteristic. Likewise, the seal joint formed by the sealingsurface 80, abutment surface 74, and seal 86 is also airtight.

The top closure plate 26 is selectively secured to the annular sealingring 66 by installing a plurality of bolts 94 through recessed apertures96 formed at evenly-spaced intervals about the perimeter of the topclosure plate 26 into correspondingly located threaded passages 98formed in the abutment surface 74 of the annular sealing ring 66. Drainholes (not shown) are provided at the base of each threaded passage 98.Referring to FIG. 6, two monitoring ports 100 are formed in the topclosure plate and are selectively sealed by plugs 102.

Attention is now directed to FIGS. 2 and 4 to describe the airtightjoints formed between the bottom closure plate 20 and the structuralshell 14 and inner shell 54. The bottom shell 20 is also configured as asolid disk. An annular flange 104 projects upwardly from the top (i.e.,inner) surface of the bottom closure plate 20, at a location spacedradially inwardly from the outer perimeter of the top closure plate 20.When the bottom closure plate 20 is placed over the bottom end of thebody 12, an upper edge of the flange 104 abuts the lower edge of theinner shell 54. The upper edge of the flange 104 is welded to the loweredge of the inner shell 54. The bottom edge 46 of the lower shellportion 18 is welded to the bottom closure plate 20. Both welds are fullpenetration welds formed about the full circumference of the bottomclosure plate 20, and the weld between the inner shell 54 and the flange104 is airtight. The weld between the lower shell portion 18 and theflange 104 is also believed to be airtight, but is not tested for thatcharacteristic.

A drain port 106 is formed through the bottom closure plate 20, from thetop (inner) surface of the plate to the plate's outer circumference, andis sealed with a threaded bolt 108 capped by a threaded plug 110. Thethreaded bolt 108 and threaded plug 110 each include a seal (not shown)that is leak tight. The port 106 permits drainage of liquids from theinterior cavity 40 of the cask 10. The drain port 106 may be located atany orientation on the bottom of the cask.

Referring to FIGS. 2 and 5A, the central access aperture 22 is formedcentrally through the bottom closure plate 20. An annular recess 112 isformed in the bottom (i.e., outer) side of the bottom closure plate 20,effectively enlarging the diameter of the bottom portion of the centralaccess aperture 22. The recess 112 defines an annular abutment surface114. A hardened inlay 116, which may be formed by weld overlay of a hardmetal, such as inconel, is formed angularly around the innermost portionof the abutment surface 114 adjoining the access aperture 22. The inlay116 is polished to define a sealing surface.

The access cover plate 24 is configured as a solid disk having an outerdiameter that is sized to be received within the recess 112. An annularrecess is formed in the top (i.e., inner) side of the access cover plate24 about the plate's perimeter to define a sealing surface 118. Anon-recessed center portion 120 is bordered by the sealing surface 118.When the access cover plate 24 is assembled to the bottom closure plate20, the access cover plate 24 is received within the recess 112 of thebottom closure plate 20, with the center portion 120 of the access plate24 being received within the central access aperture 22. The sealingsurface 118 overlies the inlay 116 in this installed configuration.

Referring to FIGS. 5A and 5B, two half-dovetailed annular grooves 122,configured similarly to the previously described grooves 84 in the topclosure plate 26, are formed in the sealing surface 118. Again, seals(not shown) are received within the grooves 122 and are compressedbetween the sealing surface 118 and the inlay 116 to form an airtightseal between the ram closure plate 24 and the bottom closure plate 20.The ram closure plate 24 is retained in place by a plurality of bolts124 inserted through recessed apertures 126 formed at spaced intervalsabout the periphery of the access cover plate 24 and received withinthreaded passages 128 formed at corresponding locations in the abutmentsurface 114 of the bottom closure plate 20.

The bottom closure plate 20 is preferably formed from a machine forging,such as a type 304 stainless steel forging. The ram closure plate ispreferably formed from a higher strength material, such as type XM-19stainless steel.

Referring to FIG. 6, the construction of the shield jacket 28 will nowbe described in greater detail. As noted previously, the outer skin 58of the shield jacket 28 is larger than the external diameter of theupper shell portion 16 and lower shell portion 18. The annular spacecreated therebetween is filled with neutron radiation absorbing shieldmaterial 60. Neutron radiation shielding material 60 is not a strongload bearing material, and thus a plurality of elongate reinforcingmembers 130 are embedded within the shield material 60. The elongatereinforcing members 130 are oriented so as to be parallel to the centralaxis 44 of the cask body 12.

Each reinforcing member 130, which are also illustrated in FIGS. 8 and9, is bent centrally along its length on two fold lines, such that eachmember 130 defines a flattened V-shaped cross section. Each member 130thus has an elongate center portion 132 and first and second elongateleg portions 134 that project angularly outwardly from the centerportion 132. The center portion 132 of each member 130 is welded to theinterior of the outer skin 58 of the shield jacket 28. The projectingedges of each of the two leg portions 134 contacts and is welded to theoutside of the structural shell 14. This gives a "corrugated"reinforcing effect to the structure of the shield jacket 28. Thereinforcing members 130 transfer heat from the structural shell 14through the shield jacket 28 to the exterior of the cask 10 to removethe decay heat of spent fuel contained within the cask 10, and alsoprovide an integral structural system for supporting the cask duringtransport.

Reference is now had to FIGS. 1, 7, and 8 to describe an additionalfeature of the cask 10. The cask 10 includes a tie-down key waystructure 136. The key way structure 136 serves as an anchor point for atie-down that secures the cask 10 to a transport skid for securetransportation. The key way structure 136 defines an elongate arcuateopening formed through the shield jacket 28 approximately mid-length ofthe body 12. The key way structure 136 has a radially oriented lengthand an axially oriented width, and is formed from four frame membersthat are welded directly to the structural shell 14.

Referring now to FIGS. 7 and 8, the long sides of the key way structure136 are formed by arcuate bearing blocks 138 that are mounted arcuatelyin spaced-apart disposition on the lower shell portion 18. The perimeterframe of the key way structure 136 is completed by two longitudinallyoriented tie-bar members 140 welded across the opposing ends of thebearing blocks 138. Each of the bearing blocks 138 and tie-bars 140 iswelded to the lower shell portion 18, and cooperatively define arectangular frame. A recess 142 is formed in the outer surface of eachof the bearing blocks 138 and tie-bars 140 about the inner perimeter ofthe frame defined thereby.

The perimeter frame defined by the bearing blocks 138 and tie-bars 140are further reinforced by an arcuate pad plate 144 that fits over thebeating blocks 138 and tie-bars 140. The pad plate 144 is disposedwithin the interior of the shield jacket 28 and is welded directly tothe lower shell portion 18, as well as to the bearing blocks 138 and tiebars 140. The outer skin 58 of the shield jacket 28 is also welded tothe tie bars 140 and bearing blocks 138.

The pad plate 144, tie bars 140, and bearing blocks 138 are preferablyformed from a high-strength metal, such as type XM-19 stainless, due tothe stress imposed on them during use. Because it is desired that thekey way structure 136 be sacrificed rather than the integrity of thestructural shell 14 in the event of excessive loads applied to the keyway structure 136, the welds between the key way structure 136 and thelower shell portion 18 and outer skin 58 of the shield jacket 28 arerelatively small. This ensures that the key way structure 136 will giveway prior to breakage of the structural shell 14 in the event of extremeloads on the key way structure 136.

The construction of the upper trunnions 30 and lower trunnions 34 willnow be described with reference to FIGS. 9 and 10, respectively. Theupper trunnions 30 and lower trunnions 34 are similarly constructedexcept as noted. Thus, only the upper trunnion 30 will be described withit being understood that the same description applies to the lowertrunnion 34. The upper trunnion 30 has a cylindrical body 146. Anannular flange 148 is formed about the midsection of the body 146. Arecess 150 is formed in one of the circular faces 152 of the body 146,and extends fully into the interior of the body 146 to define a cavity154. The body 146 thus has a hollow configuration. The portion of thetrunnion body 146 between the flange 148 and the first face 152 definesa cylindrical base 156.

The interior cavity 154 is filled with neutron radiation absorbingshield material 60. The neutron shield material 60 is capped andretained by a circular back plate 158 that is received within the recess150 and welded in position. The presence of the neutron shield material60 reduces streaming of neutrons through the upper trunnions 30.

A cylindrical beating projection 160 projects from the second circularface 162 of the trunnion body 146. An annular flange 164 is formed aboutthe end of the bearing projection 160. The bearing projection 160,flange 164, and second circular face 162 cooperatively define a beatinggroove that can be grasped by a correspondingly contoured hook fortransport of the cask 10. A plurality of apertures 166 are formedthrough the flange 48 at spaced intervals about the perimeter of theupper trunnion 30, for purposes of securement to the cask 10 by bolts168.

The lower trunnions 34 are configured similarly to the upper trunnions30, except that no cylindrical beating projection 160 projects from thetrunnion body 146. Additionally, the interior cavity 154 is not filledwith a neutron shield material, and back plate 158 is also not included.

The upper trunnion 30 can be selectively and releasably secured to thecask body 12 by engagement with the upper trunnion mounting sleeve 32.The upper trunnion mounting sleeve 32 consists of a tubular sleeve thatprojects through and is welded to the upper shell portion 16. A circularaperture 170 is formed through the upper shell portion 16 at the desiredlocation for the upper trunnion mounting sleeve 32. A similarly orientedaperture is formed through the outer skin 58 of the shield jacket 28.The upper trunnion mounting sleeve 32 is installed through the shieldjacket 28 and the upper shell portion 16 such that the central axis (notshown) of the upper trunnion mounting sleeve 32 is oriented radiallyrelative to the longitudinal axis 44 of the cask body 12.

The upper trunnion mounting sleeve 32 is welded fully about itsperimeter to the upper shell portion 16. Additionally, a weld is formedbetween the outer skin 58 of the shield jacket 28 and the upper trunnionmounting sleeve 30. A circular trunnion filler plate 171 is installedwithin the upper trunnion mounting sleeve 32, and positioned within theradially inward end of the trunnion mounting sleeve 30 so as to be inline with the arc of the upper shell portion 16. The trunnion fillerplate 170 is welded to the interior of the upper trunnion mountingsleeve 32 to seal the radially interior end of the upper trunnionmounting sleeve 32.

An annular recess 172 is formed about the entry to the upper trunnionmounting sleeve 32. To secure the upper trunnion 30 in position on thecask 10, the circular base 156 of the upper trunnion 30 is slidablyreceived within the interior passage 174 defined by the upper trunnionmounting sleeve 32, and the flange 148 of the upper trunnion 30 isreceived within the recess 172. The dimensional tolerances of theinterior passage 174 of the upper trunnion mounting sleeve 30 and therecess 172, as well as the base 156 and flange 148 of the upper trunnion30, are closely controlled such that a very close slip fit is formedbetween the upper trunnion 30 and the upper trunnion mounting sleeve 32.This ensures that the upper trunnion 30 cannot become cocked within theupper trunnion mounting sleeve 32.

The bolts 168 are installed through the apertures 166 and the flange 148of the upper trunnion 30 and into correspondingly arranged threadedpassages 176 formed into the recess 172 of the upper trunnion mountingsleeve 32.

Because of this two-piece mounting of the upper trunnion 30, utilizingthe separate upper trunnion 30 and upper trunnion mounting sleeve 32,the upper trunnion 30 can be removed as desired when hoisting of thecask 10 is not required. Additionally, because the upper trunnionmounting sleeve 32 receives and captures the upper trunnion 30, thebolts 168 are substantially isolated from shear and tensile loads, whichinstead are transmitted from the upper trunnion 30 to the upper trunnionmounting sleeve 32 and then to the structural shell 14. Thisconstruction helps to ensure that the upper trunnions 30 are not tom offof the structural shell 14 when the upper trunnions 30 are grasped tohoist the weight of the cask 10 and the contents therein.

The upper trunnion mounting sleeve 32 and upper trunnion 30 arepreferably formed from a high strength metal, such as type XM-19stainless steel. The trunnion backing plate 158 can be formed from type304 stainless steel or other suitable metals.

Referring to FIG. 10, the lower trunnion mounting sleeve 36 isidentically constructed and secured to the lower shell portion 18, aswas the upper trunnion sleeve 32 constructed and secured to the uppershell portion 16, except as noted herein. Because the stresses imposedon the lower trunnions 34 are not as great as those imposed on the uppertrunnions 30, a recess 172 is not formed in the outer face of the lowertrunnion mounting sleeve 36 to receive the flange 148 of the lowertrunnion 34. Instead, the axial length of the lower trunnion mountingsleeve 36 is correspondingly reduced, and the flange 148 of the lowertrunnion 34 abuts the annular exterior face 178 of the lower trunnionmounting sleeve 36.

Referring now to FIG. 11 A, oecen when the cask 10 has been loaded witha canister 38, the cask 10 will be temporarily stationary on-site.During such times, it is not required to mount the upper trunnions 30and lower trunnions 34 on the cask 10. In such instances, it is desiredto further reduce neutron streaming past the trunnions 30 and gammastreaming past trunnions 34 by removing the trunnions 30 and 34, andcapping the upper trunnion mounting sleeves 32 with trunnion shields 180trunnion mounting sleeves 36 with trunnion shields 181. Trunnion shields180 are metal disks that are filled with neutron shield material 60 (notshown) and bolted to the upper trunnion mounting sleeves 32. Trunnionshields 180 are solid metal disks bolted to the upper trunnion mountingsleeves.

Additionally, when not in transport, the key way structure 136 is notbeing utilized. At such times, it is desirable to mount a key way shield182 (also shown in FIG. 1) to cover the key way structure 136. The keyway shield 182 is again filled with a neutron shield material 60 and issecured by bolting a top plate 184 to the frame of the key way structure136. This again is to reduce neutron streaming through the key waystructure 136.

Finally, during unloading of the canister 38 from the cask 10, it isnecessary to remove the access cover plate 24 from the bottom closureplate 20, as shall be described briefly below. During such times whenthe access cover plate 24 is removed from the cask 10 and it is notactually necessary to insert a ram, as shall be described, through thecentral access aperture, an access aperture shield assembly 186 issecured centrally to the bottom closure plate 20 to cover the accessaperture 22.

Referring to FIG. 11B, the access aperture shield assembly 186 consistsof an annular first shield member 188 that is formed from two annularplates 190 that are secured together by an annular ring 192. An aperture194 is formed centrally through the plates 190, and an internal ring 196borders this aperture 194. The interior of the first shield member 188is filled with a neutron absorbing shield material 60. A secondsimilarly constructed shield member 198 is also utilized. Shield member198 is also formed as a disk, but is a smaller diameter than shieldmember 188, and includes no central aperture. It also is filled withneutron absorbing shield material 60. Shield member 198 is supported bya plurality of hangers 200, extending outwardly from the first shieldmember 188 around aperture 194 in the shield member 188. When both theshield member 198 and shield member 188 are utilized, the complete areaof the central access aperture 22 is shielded.

Referring to FIGS. 12, 13 and 14, in another aspect, the presentinvention relates to a skid for supporting and protecting thetransportation cask for spent nuclear fuel during transportation. FIG.12 illustrates a conventional trailer 226 that includes a transportationcask enclosed by skid 220 formed in accordance with the presentinvention and a pair of impact limiters 222 formed in accordance withthe invention described in the application entitled Impact Limiter ForSpent Nuclear Fuel Transportation Cask. In FIG. 12, the transportationcask is not visible, as it is completely encased by skid 220 and impactlimiters 222. Skid 220 is further enclosed by a curtain of expandedmetal 224, which further obscures skid 220 and the transportation cask.The curtain of expanded metal 224 is provided around skid 220 in orderto shield skid 220 and the transportation cask from sunlight. In FIG.12, the longitudinal axis of the transportation cask is parallel to thelength of trailer 226. Impact limiters 222 are positioned on oppositeends of the generally cylindrical transportation cask. Skid 220 supportsthe transportation cask along its length between impact limiters 222, asdescribed below in more detail.

Referring primarily to FIGS. 13 and 14, transportation skid 220comprises a lower supporting member 290 and an upper retaining member292. Lower supporting member 290 carries the vertical and lateral caskloads and includes a plurality of parallel spaced-apart plates 294 lyingperpendicular to the longitudinal axis of the transportation cask.Plates 294 include an outer peripheral portion that is substantiallysquare and in use rests on the bed of a transportation trailer. Theinner periphery of plates 294 includes a trough which in the illustratedembodiment is semicircular and mates with a portion of the exteriorsurface of the transportation cask. At the bottom of the trough insupporting member 290 is a saddle 291 that comprises a plate extendinglengthwise along the bottom of the trough and widthwise up the sides ofthe trough. In the illustrated embodiment, saddle 291 occupiesapproximately one-third of the bottom radius of the trough. At thebottom of the trough centrally located along the length of saddle 291 isan upward protruding rectangular block 296 that serves as a shear keyfor mating with tie-down keyway structure (136 in FIG. 1) on thetransportation cask. Block 296 cooperates with the transportation caskin order to provide an independent means for carrying axial shear loadsfor the cask. Spaced-apart plates 294 of lower supporting member 290 areconnected by a plurality of longitudinal fins 201 running parallel tothe longitudinal axis of the transportation cask. In the illustratedembodiment, plates 294 are made from one-inch steel plates and fins 201comprised of one-half-inch thick steel plates. Plates 294 providesupport for the transportation cask for downward, vertical andtransverse loads from the cask.

Upper retaining member 292 carries vertical upward loads for the caskand includes a plurality of spaced-apart plates 298 lying perpendicularto the longitudinal axis of the transportation cask. In the illustratedembodiment, the inner periphery of plates 298 includes an invertedsemi-circular trough that is a mirror image of the trough in supportingmember 290. The outer periphery of plates 298 is substantiallyconcentric with its inner periphery. Upper retaining member 292 alsoincludes a plurality of parallel longitudinal fins 202 that arepositioned parallel to the longitudinal axis of the transportation cask.In the illustrated embodiment, plates 298 and fins 202 are made frommetal, such as aluminum. Upper retaining member 292 and lower supportingmember 290 mate with each other to define a cylindrical cavity whichcompletely encases the neutron shielding material (60 in FIG. 2).

As described above, the neutron radiation Shielding material is not astrong load-bearing material, and accordingly, a plurality of elongatereinforcing members (130 in FIG. 6) are embedded within the shieldmaterial. The elongate reinforcing members are oriented so as to beparallel to the central axis of the cask body. The radial spacingbetween fins 201 and fins 202 is such that when the transportation caskis mated with rectangular block 296, the center portions 132 of theelongate reinforcing members 130 in the neutron radiation shieldingmaterial are aligned and rest along longitudinal fins 201 and 202.Accordingly, the neutron shielding material does not carry the load ofthe cask, but rather the elongate reinforcing members resting on thelongitudinal fins serves to carry the load of the cask.

Utilization of the cask 10 shall now be briefly described. When it isdesired to install a canister 38 into the cask 10, the access coverplate 24 is secured to the bottom closure plate 20, while the topclosure plate 26 is removed from the cask body 12. The canister 38 isinstalled into the interior cavity 40 of the cask body 12. Theseoperations are performed inside pools or otherwise in accordance withindustry practice. Transport of the open cask during this time is madeby grasping the upper trunnions 30 to hoist the cask 10. After water isdrained from the interior of the cask 10, and the cask 10 is dried inaccordance with standard industry practice, the top closure plate 26 issecured to the cask body 12.

The cask 10 is now hoisted by again hooking the upper trunnions 30 tomove the cask 10 to a transport trailer. While being hoisted, the cask10 is oriented vertically with the weight of the cask being supported bythe upper trunnions 30. The cask 10 is then repositioned horizontally ona trailer, during which operation the lower trunnions 34 are utilized tostabilize and reposition the cask 10. The cask 10 can then betransported to the site where the canister 38 is to be installed in ahorizontal storage module or other storage module.

Once the cask 10 has arrived at the storage site, the top closure plate26 is removed and the top end of the open cask body 12 is docked withthe intended storage module. The access cover plate 24 can then beremoved, and replaced with the access aperture shield assembly 186 toreduce neutron streaming. When it is time to transfer the canister 38from the cask 10 to the storage module, the second shield member 198 isremoved from the access aperture shield assembly 186. A ram can then beinserted through the remaining shield member 198 and the access aperture22 into the interior cavity 40 of the cask body 12. The ram then pushesthe canister 38, which slides on the rails 64 as it moves through theopen end of the cask body 12, defined by the annular sealing ring 66.The canister 38 thus moves into the storage module. Once transfer of thecanister 38 is completed, the cask 10 can be reassemble and reutilized.

For transportation, the reverse operations to those described above areperformed to retrieve the canister into the cask. The cask in thenlifted from the trailer and placed on a suitable transportation skid,such as described above, with a shear key which engages the keywaystructure 136. The trunnions 30 and 34 are removed and trunnion shields180 and 181 are installed.

While the present invention has been described above in terms of apreferred embodiment, it should be readily apparent to those of ordinaryskill in the art that various alterations, modifications andsubstitutions may be made within the scope of the present invention. Forexample, materials other than those described can be utilized to formthe components of the cask 10, provided that they meet the parametersset forth herein. It is thus intended that the scope of letters patentgranted hereon be limited only by the definitions contained in theappended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A skid for transportinga nuclear fuel transportation cask that includes a neutron radiationshielding material reinforced with elongate members, the skidcomprising:a supporting member that includes a plurality of parallelspaced-apart plates aligned perpendicular to a longitudinal axis of thecask, said parallel spaced-apart plates including a semi-circular troughfor mating with the transportation cask, the parallel spaced-apartplates of the supporting member being connected by a plurality oflongitudinal fins parallel to the longitudinal axis of the cask; and aretaining member that includes a plurality of spaced-apart platesaligned perpendicular to a longitudinal axis of the cask, the parallelspaced-apart plates including a trough for mating with thetransportation cask, the parallel spaced-apart plates of the retainingmember being connected by a plurality of longitudinal fins parallel tothe longitudinal axis of the cask, the longitudinal fins of thesupporting member being spaced apart a distance such that when the caskrests in the trough of the supporting member, the fins of the supportingmember are aligned with the elongate members that reinforce the neutronradiation shielding material to transfer the load between thetransportation cask and the skid.
 2. The system of claim 1, wherein thespaced apart plates of the support member include an outer peripherythat includes a flat base for resting on a fiat surface.
 3. The systemof claim 1, wherein the longitudinal fins of the retaining member arespaced apart a distance such that when the cask rests within the troughof the retaining member, the fins of the retaining member are alignedwith the elongate members that reinforce the neutron radiation shieldingmaterial to transfer the load between the transportation cask and theskid.